Switch to the new MingW ABI.

GCC 4.7 changed the MingW ABI. On the clang side this means that methods now
have the thiscall calling convention by default.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197164 91177308-0d34-0410-b5e6-96231b3b80d8